i2c: introduce struct i2c_dt_spec

[JordanYates]

Introduces the struct i2c_dt_spec type, which contains the complete I2C bus information derived from devicetree. It serves the same purpose as struct spi_dt_spec in that it can be constructed automatically in DEVICE_DT_INST_DEFINE macros and provided as a single handle to I2C API calls.

It encapsulates less information than struct spi_dt_spec, but it still reduces boilerplate and chances for error in device drivers.
It also allows the following pattern to be used for drivers that support both SPI and I2C buses.

struct device_config {
    union {
        struct spi_dt_spec spi;
        struct i2c_dt_spec i2c;
    };
};

[JordanYates]

If an i2c_is_ready function is desired (equivalent to spi_is_ready) I can add that.
Currently drivers need to dereference an additional layer:

int device_init(const struct device *dev) {
    const struct device_config *config = dev->config;

    if (!device_is_ready(config->bus.bus)) {
        ...
    }
}

[mbolivar-nordic]

Putting a -1 on to give me time to review, which I am doing right now.

[mbolivar-nordic]

LGTM, thanks!

struct device_config {
    union {
        struct spi_dt_spec spi;
        struct i2c_dt_spec i2c;
    };
};

Regarding this pattern from the PR description, I'm guessing you're omitting the extra ifdeffery for the sake of clarity.

When you're going to do it "for real" I assume you'll do something similar to what's in bme280.h that avoids wasting flash on room for a SPI config in the case of I2C-only systems, like this:

#define BME280_BUS_SPI DT_ANY_INST_ON_BUS_STATUS_OKAY(spi)
#define BME280_BUS_I2C DT_ANY_INST_ON_BUS_STATUS_OKAY(i2c)

union bme280_bus_config {
#if BME280_BUS_SPI
	struct spi_config spi_cfg;
#endif
#if BME280_BUS_I2C
	uint16_t i2c_addr;
#endif
};

That is, you could do

union bme280_bus_config {
#if BME280_BUS_SPI
	struct spi_dt_spec spi_spec;
#endif
#if BME280_BUS_I2C
	struct i2c_dt_spec i2c_spec;
#endif
};

And then remove the bus pointer from struct bme280_config and access it through the spec structure instead or something like that.